Integrated circuits were early encased in dual-in-line packages (DIPs), which are small, elongated plastic boxes with the exterior leads coming out on either side of the box and turning down in two parallel rows, one along each side of the package. DIPs are mounted into corresponding holes in a printed circuit board that has been plated through and to which the DIPs are soldered to make electrical and mechanical connection.
As integrated circuits became smaller and more complex, the number of leads coming out of the package increased, even though the package size remained the same or decreased. One solution to this problem was the flat, plastic leadless chip carrier (PLCC), which typically is a small, flat, square package having the leads coming out and formed around each of the four edges of the package. PLCCs are typically mounted in sockets with contacts that mate with the leads on the PLCCs.
More recent PLCCs have even been surface mounted directly on the corresponding pattern of electrical contact pads on a printed circuit board. Surface mounting of parts has gained widespread interest of late because of the ease with which the parts can be picked up and placed on the printed circuit board in contrast to a through-hole package which must be much more precisely aligned. Not only may a printed circuit board be more easily populated with ICs, but they may also be more easily removed. In addition, it is possible to achieve a higher density of ICs on a printed circuit board with surface mount packages. The leads on the packages to be surface mounted are often much shorter and better protected than leads that are mounted through holes in the printed circuit board so that there is less danger of damage to the leads during handling of surface mountable packages. However, current surface mounting techniques also have a number of disadvantages. First, a solder mask for the printed circuit (PC) board must be defined and generated. Next, the solder paste must be applied to the PC board which requires an alignment step. The actual application of solder paste is typically done using a stencil process or a screen process, as in silk screen printing, both of which involve many variables and can be complex. Next, the surface mount device must be picked up and positioned onto the PC board, which has two patterns thereon, the PC artwork and the solder mask artwork. This positioning requires a second alignment step. Finally, the solder undergoes reflow to make the surface joint by either a vapor phase or an infrared (IR) treatment. It would be advantageous if one or both of the alignment steps could be simplified or eliminated.
One innovative way of surface mounting leadless chip-carriers is described by Jerry Lyman in "Frame Permits Use of Glass Boards," Electronics, June 28, 1984, pp. 54 and 56. This article describes the use of a square foam frame with solder columns embedded at intervals that correspond to the leads on the chip carrier. During the soldering operation, the foam frame dissolves. However, as integrated circuits became even more complex, even the PLCC has become inadequate to carry all the leads required without making the package unreasonably large.
The most recent solution to the problem of putting a large number of leads on a small package is the pin grid array (PGA). These packages are generally flat and square, but have a large number of leads coming out of the bottom of the package, parallel to each other, perpendicular to the bottom of the package. The PGA packages can contain pin counts on the order of 100 in a 10.times.10 array or 169 in a 13.times.13 array or even larger. Discussions of PGA-type packages may be found in U.S. Pat. No. 4,513,355 to Motorola entitled "Metallization and Bonding Means and Method for VLSI Packages," and in J. W. Balde, et al. "Alternatives in VLSI Packaging," VLSI Design, December, 1983, pp. 23-29. However, one drawback to PGAs is that they require through-hole mounting in the printed circuit boards due to the high number of leads. A means for surface mounting PGA packages would be welcomed as an advance in the packaging art.